In alternating current, how often does the current alternate between negative and positive? 120 times per second 60 times per second 90 times per second 180 times per second

Power systems operate at different frequencies depending on the region and application: 50 or 60 Hz (power grids) and 400 Hz (in airplanes).

For one cycle lasting one period

T

[seconds], the frequency [Hz] is simply = 1/

f

=

1

/

T

which is the number of cycles per second. At 50 Hz, there are 50 cycles in one second with each cycle lasting only = 1/50=0.02

T

=

1

/

50

=

0.02

seconds or 20 milliseconds.